Generally, a polyester is a linear polymer synthesized from a dicarboxylic acid or an ester-forming derivative thereof and a diol or an ester-forming derivative thereof. However, as described in, for example, Non-Patent document 1, a cyclic compound is generated in a linear polymer based on the equilibrium. It is known that the cyclic compound is a low-molecular substance, it is precipitated in a polyester forming process, or precipitated on a surface of a product freely in time, in particular, it is much extracted in a forming process or a processing process or in a case where a solvent is used at the time of using a product, and it causes various troubles.
For example, in a case of a base film for magnetic recording media, large protrusions are formed by the precipitation of cyclic compound and the magnetic recording may be damaged, or in a case of an optical base film, the cyclic compound is precipitated by heating in a processing process and it may generate an optical turbidity or a calescence point, or in a case of a film laminated with a steel plate, a bloom may be generated in the appearance after lamination to the steel plate.
These cyclic compounds have been informed variously, and for example, as to polyethylene terephthalate, it is informed in, for example, Non-Patent document 2, that the main component of the cyclic compound is a cyclic trimer and it is generated by an equilibrium reaction at the time of condensation polymerization of polyethylene terephthalate.
For the above-described problem, for example, in Patent documents 1 to 10, a method for a solid phase polymerization of a polyester resin at a temperature of not higher than the melting point at a reduced-pressure condition in an inert gas atmosphere is proposed.
However, in such a technology wherein the cyclic compound is reduced by the solid phase polymerization, although the amount of the cyclic compound is certainly reduced, because at the same time the condensation polymerization of the polyester progresses and the rise of the polymerization degree becomes great, the prepared material is not suitable as a raw material for producing a formed product little in content of cyclic compound.
Namely, if the polymerization degree of polyester becomes great, at the time of forming, the viscosity of the molten polymer becomes high, and therefore, the load at the time of extrusion becomes large or the polymer temperature rises by exothermic ascribed to shear, and a problem such as thermal decomposition occurs.
In order to solve such a problem, proposed are a method for adjusting the pressure reduction degree at the time of solid phase polymerization (Patent document 11), a method for adjusting the flow rate of inert gas (Patent document 12), a method for heating by an inert gas atmosphere containing a glycol component (Patent document 13) and further a method for heating under a condition of an inert gas atmosphere after removing fine or film-like substances (Patent document 14). Even in these methods, however, because the polymerization degree of the polyester obtained varies, it has been very difficult to obtain a polyester with a constant quality industrially.
Further, although Patent document 15 describes a technology for reducing a cyclic compound by heat treatment in an inert gas atmosphere, even in this method, the amount of a cyclic trimer can be certainly reduced, but there is a problem that the polymer is colored.
Non-Patent document 1: “Polymer Chemistry” written by P. J. Flory, (translated by Oka and Kanemaru, published by Maruzen Publisher Corporation) p90-97
Non-Patent document 2: “Saturated Polyester Resin Hand Book” written by Kazuo Yuki, (Nikkan Kogyo Newspaper Corporation) p167-178
Patent document 1: Japanese Patent 3418987
Patent document 2: JP-A-2003-306538
Patent document 3: JP-A-2001-172372
Patent document 4: JP-A-11-199660
Patent document 5: JP-A-9-249744
Patent document 6: JP-A-9-59362
Patent document 7: JP-A-6-234834
Patent document 8: JP-B-51-48505
Patent document 9: JP-A-49-101462
Patent document 10: JP-A-53-101092
Patent document 11: JP-A-55-89331°
Patent document 12: JP-A-55-89330
Patent document 13: JP-A-8-120062
Patent document 14: JP-A-2002-173528
Patent document 15: Japanese Patent 2745676